1. Field of the Invention
The present invention relates to a rotary cutting tool and a method for manufacturing an iron golf club head using the rotary cutting tool.
2. Prior Art
Most of today's iron golf club heads have a cavity that is formed on the back of the club head. The cavity is provided so as to increase the inertial moment and depth of the center of cavity by reducing the overall weight of the head, by lowering the center of gravity, and by distributing the weight to the periphery.
These iron golf club heads which are generally called cavity irons are manufactured by way of forging, lost-wax processing, etc. In addition to these methods, a cavity iron head is manufactured by, for instance, a method shown in FIG. 7. In this method, a cavity is formed by cutting an indentation 2 on the back side of a head ingot 1 which is roughly finished and formed by forging, and this cutting is performed by a rotary cutting tool 10 which is controlled by an NC machining system.
However, in the conventional method as described above, the cutting section 10a of the rotary cutting tool 10 has a cylindrical shape, and the angle .theta. of the cutting surface is perpendicular to the horizontal plane. Accordingly, as seen from FIG. 8, the inner circumferential surface 3a on the sole side, the inner circumferential surfaces 3b on the toe and heel sides, and the inner circumferential surface 3c on the top side of the head (these surfaces are continuous around the entire circumference of the indentation 2) can only be cut into the shape of an upright surface that has a cutting angle of .alpha. which is equal to the cutting surface angle of .theta. of the cutting tool.
On the other hand, a rotary cutting tool 20 which has a cutting section 20a of a conical shape may be used in the above method. The tool, as seen from FIG. 9, has an acute cutting surface angle .theta. with respect to the horizontal plane and can grind the inner circumferential surface 3 of the indentation 2 of the head ingot 1 into an overhanging surface shape. However, with this cutting tool 20, the inner circumferential surface 3 of the indentation 2 can only be cut into an overhanging surface shape which has a cutting angle of .beta., that is equal to the cutting surface angle .theta. of the cutting tool 20, around the entire circumference as shown in FIG. 10. Furthermore, the comer edge portions of the inner circumferential surfaces 3b on the toe and heel sides and the inner circumferential surface 3c on the top side are sharp and that causes possible danger in the handling of the club (e. g., cutting the fingers). In addition, the club made by this cutting tool 20 tends to be easily chipped due to outside impacts.
Furthermore, when the inner circumferential surface 3c on the top plate side of the indentation 2 has an overhanging surface shape, the weight of the protruding corner edge portion tends to prevent the lowering of the center of gravity of the head.
In view of these problems, it is possible to cut the head ingot 1 by selectively using two rotary cutting tools: one with a cylindrical cutting section 10a, and another with a conical cutting section 20a. However, with this method, finishing work cannot be accomplished in a single continuous process, thus requiring considerable time and effort and resulting in poor production efficiency.
Furthermore, because of the changeover between the rotary cutting tool 10 and rotary cutting tool 20, a boundary a is formed between the overhanging and upright surfaces of the indentation 2 as a step on the inner circumferential surfaces 3b on the toe and heel sides. Thus, the inner circumferential surface 3 of the indentation 2 become discontinuous; and as a result, the cavity has an unattractive appearance.
The step of such a boundary a between the overhanging and upright surfaces on the innner circumferential surface 3 of the indentation 2 can ordinarily be camouflaged by a pattern of indentations and projections. However, since the position of the step is determined by the shape of the cavity, there are limits to the position and shape of such a pattern of indentations and projections.